WASHINGTON (CNN) — The U.S. government has warned airlines to pay particular attention to the possibility of terrorists attempting to hide explosives in shoes, a result of new intelligence, according to two people familiar with the situation.

The officials stressed there is no specific threat or known plot.

Intelligence collected by the United States and other countries has indicated terror groups have been working on new shoe-bomb designs, the sources said.

That knowledge prompted the warning to airlines to be on the lookout for possible explosives hidden in shoes on flights from overseas to the United States.